Various semiconducting metal oxides have been used in conjunction with a variety of metal and non-metal additives in the fabrication of gas sensitive films suitable for use in gas detection apparatus. Exposure of such gas sensitive films to the gas of interest generally is detected as a change in conductivity of the film. In general, these prior devices exhibited inherent deficiencies in sensitivity, selectivity, response and recovery times, and/or calibration stability. The electrical characteristics and subsequent gas response characteristics of such materials when employed as gas sensors in previous gas sensing equipment have been found to be highly dependent upon film properties such as thickness, uniformity of composition, purity, film porosity, and density. Since it has previously been difficult to adequately control the foregoing factors this art has been seeking a technique of fabrication which would be capable of producing films with the above mentioned and other properties well controlled. In addition it is of course desireable that any new technique should be reproducible and cost effective. Further, the previous sensors were sometimes of limited utility if they were not capable of low temperature operation. This property is advantageous when sensing flammable gases in that there would be a reduced hazard of flammable gas ignition by the operating sensor, as well as an increased realiability and sensor life, reduced sensor power requirements, and better compatibility with on-chip integrated signal processing circuitry.
The previous attempts to achieve the foregoing properties employed several deposition techniques for depositing the materials and combinations of materials found suitable for use in semiconducting gas sensors. Typically the fabrication methods employed have included sintering, vacuum evaporation, sputtering, chemical vapor deposition, pyrolytic spray deposition, and solution coating. Besides the previously mentioned drawbacks, each of the foregoing methods creates specific problems. For example, sintered films often lack sensitivity due to lack of porosity in the processed material. Vacuum evaporation, sputtering, and chemical vapor deposition processes are costly, and sometimes lack flexibility by making it difficult to properly control the introduction of certain dopants.
In practice, the spray pyrolysis techniques consist of spraying a solution containing a soluble salt of the cation of interest with the aid of a carrier gas, onto a heated substrate whereupon the solution undergoes a chemical reaction to form the resultant film. This process is characterized by relatively high substrate temperatures during deposition; e.g. several hundred degrees Centigrade. The lower limit of substrate temperature is dictated by the required chemical dissociation reaction. To be successful there must be complete dissociation of the salt and this reaction rate therefore imposes a limitation on the deposition rate.
During film formation, film uniformity can be critically influenced by spray turbulence, lateral gas flow across the substrate and boundary layer formation in the vicinity of the substrate itself. Substrate temperature control is also very critical for film uniformity. Care must be taken to minimize thermal shock which accompanies the spraying of the material onto the heated substrate. Other deposition parameters have also required close control. The carrier flow rate affects the size and velocity distribution of droplets in the spray which affects the dynamics of impingement. These and other factors inherent in this process have resulted in increased process complexity and cost. Additionally, resultant films produced by this process are generally characterized by the presence of large grain sizes which results in low resistivity which further limits their usefulness for gas sensing applications. The process is also limited in its application to only those materials which can undergo the appropriate dissociation reaction, to produce the desired product on the heated substrate.
Solution coating techniques are more widely used for gas sensor fabrication because of the simplicity of the process and suitability of the film properties. Small grain size films of high porosity are possible to achieve. A solution containing the materials in suspension and/or in the form of a soluble salt is applied by brush or dipping to a suitable heated substrate where at a temperature of typically 100 degrees C. to 200 degrees C. the volatile components are driven off. The resultant substrate and film are then partially sintered by firing at a higher temperature typically 600 degrees C. to 800 degrees C. Enhanced film porosity is often achieved by addition of materials which volatilize and evaporate from the film during high temperature firing. Examples of such materials are starch, wax, stearic acid, and silica gel.
The solution coating process is operator intensive and technique sensitive. For these reasons solution coating is not suitable for batch processing and does not produce uniform product. For example, film thickness, grain size, chemical composition uniformity and porosity, all can vary which results in non-uniform gas sensing properties within the film and from sensor to sensor. Further, certain desireable additives, particularly transition metals such as platinum or palladium introduced to the source solution as soluble organometallic salts, have the tendency to precipitate out of or localize within a static solution, presumably because of solubility limits or due to chemical reaction and are therefore difficult to use when attempting to make a uniform product. Additionally, contamination in the final film by the anion of the soluble salt is an undesired result in both solution coating and pyrolytic spray processes.
Sensor materials which have been used in the past for gas sensitive films include a number of semiconducting metal oxides, such as SnO.sub.2, ZnO, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3, and In.sub.2 O.sub.3. Examples of combinations of these materials with other materials for specific gas sensing applications are presented in the patent issued to Barry Bott Feb. 11, 1975, U.S. Pat. No. 3,865,550. The theory of operation generally proposed for these materials involves an electrochemical reaction of the gas with the solid surface of the heated sensor. The result of this reaction is to produce a charge transfer wherein an increase or decrease in the number of mobile carriers in the material takes place. In such a way, the conductivity in surface layers and at intergrannular contacts in the film is changed. This is measured usually as a change in conductance proportional to the gas concentration. Accordingly, it is highly desirable for sensors to have a large film surface area to volume ratio in order to exhibit the requisite sensitivity. While previous sensors with a high degree of porosity have had high sensitivity, such devices inherently are not selective and require operation at elevated temperatures usually above 250 degrees C. to preserve acceptable response and recovery times.
Boardman, Jr. et al. U.S. Pat. No. 3,901,067 issued Aug. 26, 1975, have described a sensor for selective detection of H.sub.2 S in air. This sensor has been shown to operate selectively at somewhat lower temperature and within a narrow range. Other similar thin film sensors are commercially available. Such sensors generally suffer from poor response and recovery times, generally on the order of several minutes. When operated at higher temperatures, sensors of this type become electrically unstable, lose selectivity, and are short-lived.
None of the above sensors combine desireable characteristics of fast response and recovery times with selectivity and stability at low operating temperatures. Response times of a few seconds and comparable recovery times are necessary for most gases, particularly H.sub.2 S because of its toxicity.
Where fast response and recovery times are achieved at higher operating temperatures (above the temperatures where thin film sensors usually operate), film stability considerations become more important. Thick film sensors fabricated by solution coating processes generally satisfy stability requirements. Sensitivity in these films is enhanced by a high degree of film porosity. However, selectivity has previously been lacking in thick film sensors.